1. Field of the Invention
The present invention relates to a thin film solar cell, and more particularly, to a method of manufacturing a thin film solar cell that prevents electrical short problems occurring in a patterning process using a laser.
2. Discussion of the Related Art
In general, solar cells are classified into various types according to a material of a light-absorbing layer. Solar cells may be categorized into silicon solar cells having silicon as a light-absorbing layer, compound thin film solar cells using CIS(CuInSe2) or CdTe, III-V group solar cells, dye-sensitized solar cells, and organic solar cells.
Among the solar cells, silicon solar cells include crystalline solar cells and amorphous thin film solar cells. Bulk-type crystalline solar cells are widely used. However, the crystalline solar cells have increasing production costs due to expensive silicon substances and complicated manufacturing processes.
Recently, by forming a solar cell of a thin film type on a relatively low cost substrate, such as glass, metal or plastic, instead of a silicon wafer, researches for reducing the production costs have been made.
A thin film solar cell according to the related art will be explained hereinafter with reference to accompanying drawings.
FIG. 1 is a cross-sectional view of a thin film solar cell according to the related art. In FIG. 1, the related art thin film solar cell 5 includes a substrate 10 of glass or plastic. A transparent conductive layer 30 is formed on the substrate 10 at each of unit cells C. A light-absorbing layer 40, which sequentially includes a p-type amorphous silicon layer 40a, an i-type amorphous silicon layer 40b and an n-type amorphous silicon layer 40c, is formed on the transparent conductive layer 30. A reflection electrode 50 is formed on the light-absorbing layer 40 at each of the unit cells C.
Although not shown in the figure, the substrate 10 including the transparent conductive layer 30, the light-absorbing layer 40 and the reflection electrode 50 thereon may face and be attached with a counter substrate, on which a polymeric material layer and an adhesive layer are sequentially formed.
Here, the reflection electrode 50 is formed of one selected from a conductive material group including a material that has relatively high reflectance, such as aluminum (Al) and silver (Ag). The reflection electrode 50 maximizes scattering properties of light passing through the light-absorbing layer 40.
In the thin film solar cell 5, light incident on the first substrate 10 passes through the first substrate 10 and the p-type silicon layer 40a and is absorbed by the i-type silicon layer 40b. Electrons and holes are generated in the i-type silicon layer 40b due to the light having a larger energy than a band gap energy of silicon. The electrons and the holes in the i-type silicon layer 40b are diffused to the p-type silicon layer 40a and the n-type silicon layer 40c, respectively, due to an internal electric field and are provided to an external circuit through the transparent conductive electrode 30 and the reflection electrode 50, respectively. According to this, solar energy can be converted into electrical energy.
A method of manufacturing a thin film solar cell will be explained hereinafter in more detail with reference to accompanying drawings.
FIGS. 2A to 2E are cross-sectional views of illustrating a thin film solar cell in steps of a method of manufacturing the same according to the related art.
In FIG. 2A, unit cells C are defined on a substrate 10 of glass or plastic. A first transparent conductive material layer 32 is formed on the substrate 10 by depositing one selected from a transparent conductive material group including oxide. The transparent conductive material group may include indium tin oxide (ITO), tin oxide (SnOx) and zinc oxide (ZnOx). The first transparent conductive material layer 32, beneficially, has a thickness larger than 5,000 Å. The first transparent conductive material layer 32 may be formed by a sputtering method. The first transparent conductive material layer 32 may be formed by a spraying method. Namely, the first transparent conductive material layer 32 may be applied or printed on the substrate 10 by spraying or injecting a sol-gel solution including a transparent conductive oxide material.
In FIG. 2B, a first laser processing apparatus (not shown) is disposed over the substrate 10 including the transparent conductive material layer 32 of FIG. 2A. The transparent conductive material layer 32 of FIG. 2A is patterned by a first laser cutting process using the first laser processing apparatus, and transparent conductive layers 30 are formed in the unit cells C, respectively. The transparent conductive layers 30 are spaced apart from each other with a constant distance therebetween due to a first separation line SL1 having a first width w1. A laser beam of the first laser processing apparatus, beneficially, has a wavelength of 1064 nm. After the first laser cutting process, the substrate 10 is exposed to correspond to a space between adjacent transparent conductive layers 30, that is, the first separation line SL1.
In FIG. 2C, a light-absorbing material layer (not shown) having a p-i-n structure is formed on the substrate 10 including the transparent conductive layers 30 by sequentially depositing a p-type silicon layer 40a, an i-type silicon layer 40b and an n-type silicon layer 40c. 
Next, a second laser processing apparatus (not show) is disposed over the substrate 10 including the light-absorbing material layer. The light-absorbing material layer is patterned by a second laser cutting process using the second laser processing apparatus, and light-absorbing layers 40 are formed in the unit cells C, respectively. The light-absorbing layers 40 are spaced apart from each other due to a second separation line SL2 having a second width w2. The second separation line SL2 is dislocated with the first separation line SL1. It is desirable that a laser beam of the second laser processing apparatus has a wavelength of 532 nm.
In FIG. 2D, a reflection material layer 52 is formed on the substrate 10 including the light-absorbing layers 40 by depositing one selected from a conductive material group including aluminum (Al) and silver (Ag) that have a relatively high reflectance. The reflection material layer 52 may be deposited by a sputtering method.
In FIG. 2E, a third laser processing apparatus (not snow) is disposed over the substrate 10 including the reflection material layer 52 of FIG. 2D. The reflection material layer 52 of FIG. 2D is patterned by a third laser cutting process using the third laser processing apparatus, and reflection electrodes 50 are formed to correspond to the unit cells C, respectively. The reflection electrodes 50 are spaced apart from each other with a constant distance therebetween due to a third separation line SL3 having a third width w3. The third separation line SL3 is dislocated with the first and second separation lines SL1 and SL2. A laser beam of the third laser processing apparatus, desirably, has a wavelength of 532 nm. At this time, the light-absorbing layers 40 under the reflection material layer 52 of FIG. 2D are also patterned by the third laser cutting process. The transparent conductive layer 30 is exposed to correspond to the third separation line SL3.
Although not shown in the figure, when the third laser cutting process is performed, the cutting between adjacent unit cells C is not perfectly made in a periphery of the substrate 10 because the laser is unfocused. To solve the problem, a fourth laser cutting process may be carried out along a direction perpendicularly crossing a direction of the third laser cutting process. The reflection material layer 50, the light-absorbing layer 40 and the transparent conductive layer 30 are cut through the fourth laser cutting process, and a portion in the periphery of the substrate 10 can be isolated. Here, a laser beam of the fourth laser cutting process may have a wavelength of 532 nm or 1064 nm.
Accordingly, the related art thin film solar cell can be manufactured.
In the thin film solar cell 5, since the light-absorbing layer 40 is formed of a silicon material having a melting point of more than 1000 degrees of Celsius, the reflection electrode 50 is formed of one selected from a conductive material group including aluminum and silver that have a relatively low melting point, for example, 660 degrees of Celsius.
The third laser cutting process is to cut the light-absorbing layers 40 and the reflection material layer 52 at a time. When the laser beam is irradiated, instant heats can be produced. The heats are carried to the reflection material layer 52, and the reflection material layer 52 may be melted. Accordingly, the reflection electrodes 50 and the transparent conductive layers 30 may be electrically connected to each other, and energy conversion efficiency may be lowered due to electrical short.
FIG. 3A is a view of enlarging the area A of FIG. 2E, and FIG. 3B is a picture of showing the area B of FIG. 3A.
In FIGS. 3A and 3B, the transparent conductive layer 30 and the light-absorbing layer 40 are formed in each unit cell C of FIG. 2E. The reflection electrodes 50 are formed on the transparent conductive layer 30 and the light-absorbing layer 40 and are spaced apart from each other by the third separation line SL3.
When the reflection material layer 52 of FIG. 2D and the light-absorbing layers 40 are cut together using the laser, the reflection material layer 52 of FIG. 2D is melted by heats due to irradiation of the laser beam, and there occurs electrical short between the reflection electrodes 50 and the transparent conductive layer 30 in the third separation line SL3. The electrical short rapidly decreases the energy conversion efficiency of the thin film solar cell.
More particularly, in the thin film solar cell including the light-absorbing layers 40 that are divided by the respective unit cells, the unit cells may be connected not in series but in parallel due to the electrical short, or the electrons and holes generated in the light-absorbing layers 40 may be reunited to decrease the concentration of carriers. Accordingly, the energy conversion efficiency is sharply impeded.